We converted 200 stable renal transplant patients (TXP) from Sandimmune (SAND) to Neoral (NEO) and adjusted dose to maintain a similar trough blood cyclosporine level. Mean age of the TXP was 45.6 +/- 11 years, most (60%) were men, 30% were African-Americans (AAs). Daily (QD) SAND was used in 68% TXPs who converted to BID NEO. A subgroup of 43 TXP on BID SAND was selected because they agreed to blood drawing for 3-hr area-under-the-curve (AUC) studies, before and two months after conversion to NEO. The subgroup had 39% AAs. The entire group showed a 21% (p<0.001) dose reduction by one month with an (p<0.01) increase in trough level -- but marked variability (range = 25% increase to 67% decrease in dose). Logistic regression analysis evaluated factors associated with dose reduction: race, sex, age, BID vs QD regimen and total dose. Two factors were statistically significant predictors: Patients with SAND dose 3 4mg/kg, were 4.9 times more likely to require dose reduction and patients on QD were 2.4 times more likely. Mean reduction in dose was 25% for the QD TXP and 13% for the BID TXP. Interestingly, race was not a predictive factor, even though AAs are reported to be poor absorbers of SAND. Comparison of AAs with Whites in the AUC subgroup showed that AAs and Whites entered the study with similar SAND doses (mg/d) and 12 hr-trough levels. AUC measurements suggested that 17 AAs had better absorption of SAND than 26 whites. Both groups improved after conversion to NEO, with earlier peak levels and higher maximal levels. Whites had a slighter, but not statistically significant, higher AUC after conversion. We conclude: 1) conversion of TXP from SAND to NEO requires a dose reduction to achieve similar trough levels, 2) a high (3 4 mg/kg) SAND dose before conversion is most predictive of need for dose reduction; converting from QD SAND to BID NEO is also predictive, and 3) there is no statistical support for the hypothesis that AAs and Whites differ in absorption of SAND or NEO - both improved after conversion.